The line on the graph shows how the atmospheric temperature will change is D. Line D
The atmospheric temperature will continue to increase slowly even after the level of atmospheric carbon dioxide holds steady at 380 parts per million (ppm). This is because there is a lag time between when carbon dioxide is emitted into the atmosphere and when it has its full effect on the climate.
This lag time is due to the fact that the oceans, which cover about 71% of the Earth's surface, absorb a lot of heat. So, even if the level of atmospheric carbon dioxide were to stop increasing, the oceans would continue to warm, which would continue to warm the atmosphere. This is why the atmospheric temperature is expected to continue to increase even after the level of atmospheric carbon dioxide holds steady at 380 ppm.